A rapid and quantitative method of imaging the in-vivo distribution of radionuclides in 3-dimensions provides unique and valuable information for studying biochemical function. A gamma camera based upon a new concept in collimation is proposed. Unlike conventional gamma cameras which utilize two-dimensional mechanical collimation for imaging single gamma emitting isotopes, and consequently have low sensitivity since a large portion of the gamma flux is absorbed by the lead septa, the proposed system provides at least a factor of 20 gain in sensitivity by using electronic collimation. Collimation is achieved electronically from a sequential interaction of the gamma radiation with two position and energy sensitive detection systems. Radiation is scattered from the first detector onto the second detector, and counts are recorded in a "coincidence" mode between the two detectors. Using the physics of Compton scattering, the gamma emitting activity is therby localized on conical surfaces within the body. Computer techniques are then used to reconstruct the three-dimensional emission images from these data. The proposed design consists of an array of germanium detectors (first detector) placed in close proximity (about 5 cm) to a standard gamma camera (second detector) without its collimator. A prototype subsystem consisting of a 4x4 array of germanium detectors will be studied. Subsequently it will be expanded to a 16x16 array and eventually to a 32x32 array. A microcomputer based data acquisition system will be developed to record coincidence counts between the germanium array and a gamma camera. Computer simulation studies and studies with measured data will be conducted to develop an algorithm to reconstruct the 3-dimensional emission image from a sampling of gamma emitting activity on conical surfaces - such as would be produced with this system. Preclinical evaluation studies will be conducted with test-objects and dogs.